The Dewey decimal system is really only used for non-fiction books, and since Huckleberry Finn is fiction, it wouldn't be represented. Your best bet is to just look in the fiction section under "T" for Twain, Mark.
Mark Twain's Huckleberry Finn would typically be found in the 800-899 range in the Dewey Decimal system, specifically in the 813 area for American literature.
800-899